There is more than one method of coating acrylic adhesive on labels. Two common methods are direct coating and indirect or transfer coating.
Direct coating is the process of placing acrylic adhesive on a back side of a face stock before the face stock is laminated to a liner. One problem with direct coating is that a certain amount of the acrylic adhesive is absorbed into the face stock. Thus, it is frequently necessary to apply extra acrylic coating to the face stock so that, when the acrylic adhesive dries, a sufficient amount of acrylic coating remains on the face stock to provide an acceptable tack level—i.e., to allow the label to be properly laminated or stuck to the liner. The use of extra adhesive adds to the cost of the process. Thus, it is desirable to place the acrylic adhesive on the silicone liner so as to reduce the amount of adhesive needed in the coating process.
Indirect coating or transfer coating is the process of coating the adhesive onto a silicone liner and then laminating the face stock onto the adhesive coated silicone liner when the adhesive is almost dry. The acrylic adhesive will then transfer to the face stock more easily, because the adhesion of the acrylic adhesive to the face stock is greater or stronger than the adhesion of the acrylic adhesive to the liner. One of the problems with indirect coating is that silicone does not have a high enough surface tension (dyne level) to allow the adhesive to properly adhere to the silicone, making it extremely difficult to use a forward coating process. As a result, the preferred coating methods are mire rod coating or reverse roll coating from a gravure cylinder or three or more reverse rollers. However, in these processes, it is quite difficult to control the coating thickness when using reverse rollers or three-roll systems, and the reverse gravure method generates work or shear hardening issues, because the adhesive tends to harden or separate prematurely in the roller cells, causing the coating to stay in the print station instead of transferring to the silicone liner.
It is, thus, desirable to provide an indirect or transfer coating process of coating adhesive onto a silicone liner that eliminates or reduces shear hardening of the acrylic adhesive.